1. Field of the Invention
This invention relates to a photosensor. The photosensor of this invention can be used for a solid-state imager, a line sensor, a solar battery etc.
2. Description of the Prior Art
As the construction of the photoelectric conversion portion of a photosensor which is applied to a solid-state imager, a line sensor etc., there has been known a structure in which a photoconductive layer is formed in a manner to overlie and cover metal electrodes disposed on a body.
A typical example of such photosensor is a solid-state imager in which the photoconductive layer forming an imaging face is arranged so as to cover the semiconductor body formed with switching elements, scanning circuitry etc. for taking out electric signals from photoelectric elements arrayed in the shape of a matrix.
The principle of the construction of the photoelectric conversion portion is illustrated in FIG. 1.
Referring to FIG. 1, numeral 1 designates a transparent electrically-conductive layer on a light projecting side, numeral 2 a photoelectric conversion layer whose basic material is amorphous Se, numeral 3 a metal electrode which is opposite to the light projecting side, numeral 4 a voltage source which functions to drive the photoelectric conversion layer, and numeral 5 a switch which typifies a scanning circuit. Since the photoelectric conversion layer is, in general, made of a high-resistivity material, the photoelectric conversion portion shown in FIG. 1 can be regarded as a capacitive device.
By closing the switch 5 for an instant under the state under which light is not projected, negative charges -Q corresponding to the supply voltage 4 are induced on the metal electrode 3. While the switch 5 is subsequently open, holes 7 generated in the photoelectric conversion layer 2 by thermal excitation or by light projection 6 travel towards the metal electrode 3, and positive charges q (q&lt;&lt;Q) are induced on the metal electrode 3. Ultimately, the charges on the metal electrode 3 amount to -Q +q. When the switch 5 is subsequently closed, negative charges 8 of -q are supplied from the voltage source 4 towards the metal electrode 3 in order to restore the charges on the metal electrode 3 to -Q corresponding to the supply voltage 4. The charge migration at this time is externally detected as a signal.
In the above process, when a surface insulating oxide film 3' overlies the metal electrode 3 as shown in FIG. 2, the holes 7 generated in the photoelectric conversion layer 2 are stored on the side of the surface oxide film 3' closer to the photoelectric conversion layer 2. Upon the subsequent closure of the switch 5, a high electric field is applied to the surface oxide film by the negative charges 8 supplied from the voltage source 4, and the negative charges 8 penetrate into the photoelectric conversion layer 2 owing to the tunneling effect. The penetrating negative charges 8' cannot cancel the stored holes 7', with the result that the dark current of the device increases and that the photo-response thereof deteriorates.
Examples of the solid-state imagers described above are disclosed in Japanese Published Unexamined Patent Application No. 51-10,715 (HITACHI), etc.